1. Field of the Invention
The present invention relates to a method for peak value reduction and in particular to methods for peak value reduction in single-carrier modulated digital transmitter signals and for peak value reduction in multi-carrier modulated digital transmitter signals.
2. Description of the Prior Art
In the transmission of digital transmitter signals the ratio of signal peak power to the average power of the transmitter signal, also called the crest factor, is of great interest. The crest factor is of particular importance for high-power amplifier output stages, such as those used in radio broadcasting. Every reduction in the crest factor results in an increase in the energy output and thus to reduced operating costs and purchase price for the amplifier output stage, since this can be operated with a smaller back-off, i.e. nearer to its optimal working point.
Methods for reducing a peak value are known from the prior art only for transmitter signals which have been modulated using multi-carrier modulation methods. Only the case of multi-carrier modulation using a fast Fourier transform (FFT) is referred to from the German patent application 19635813 entitled xe2x80x9cVerfahren zur Reduktion des Spitzenwertfaktors bei digitalen xc3x9cbertragungsverfahrenxe2x80x9d.
Similar methods which are concerned with the reduction of the peak value factor in digital transmitter signals which have been modulated using multi-carrier modulation methods are known from e.g. M. Pauli and H.-P. Kuchenbecker xe2x80x9cReduzierung der durch Nichtlinearitxc3xa4ten hervorgerufenen Auxcex2erbandstrahlung bei einem Mehrtrxc3xa4gerverfahrenxe2x80x9d, ITG-Fachbericht 136 xe2x80x9cMobile Kommunikationxe2x80x9d, A. Jones, T. Wilkinson and S. Barton xe2x80x9cBlock coding schemes for reduction of peak to mean envelope power ratio of multi-carrier transmission schemesxe2x80x9d, Electronic Letters, December 1994, D. Wulich, xe2x80x9cReduction of peak to mean ratio of multi-carrier modulation using cyclic codingxe2x80x9d, Electronic Letters, February 1996, M. Friese xe2x80x9cMulticarrier modulation with low peak-to-average power ratioxe2x80x9d, Electronic Letters, April 1996 and A. Kamerman and A. Krishnakumar xe2x80x9cReduction of peak to average power ratio for OFDMxe2x80x9d, European Patent Application No. 95306079.5 of Aug. 31, 1995.
The prior art cited above does not therefore disclose activities of any kind concerned explicitly with the reduction of the peak value in transmitter signals which have been modulated using one carrier or a plurality of carriers. In the so-called single-carrier method, i.e. a method in which the digital transmitter signal is modulated using one carrier, at most the modulation using offset quadrature phase shift keying (O-QPSK) can be regarded as an attempt at peak value reduction, but O-QPSK does not lead to a satisfactory reduction in the peak value.
The publication by S. Sheperd, J. Orriss and S. Barton, xe2x80x9cAsymptotic Limits in Peak Envelope Power Reduction by Redundant Coding in Orthogonal Frequency-Division Multiplex Modulationxe2x80x9d, IEEE Transactions on Communications, vol. 46, no. 1, January 1998, pp. 5-10 contains a supposition concerning the asymptotic behaviour of the peak value reduction using a coding, but it is not possible to discover from this publication anything concerning the nature of such codes. Furthermore, this article does not contain any indication as to how such codes are to be constructed nor an example of such a code.
In addition reference is also made to the publication John G. Proakis xe2x80x9cDigital Communicationsxe2x80x9d, 2nd edition 1989, McGraw-Hill, p. 532ff, which deals with digital transmission.
U.S. Pat. No. 5,621,762 describes a peak value suppression in which the digital data are mapped onto a constellation diagram to generate data symbols. In a scaling stage those symbols which would lead to the peak value being exceeded in a subsequent filter step in the transmit pulse filter are changed (distorted). Only those distortions of the output signal are allowed which lie within a circle of small radius around the original point, so that an error-free reconstitution of the transmitter signal can be achieved at the receiver. Consequently it is necessary to achieve an error-free reconstitution of the primary data, which leads to a severely restricted choice of possible output signal distortions.
EP 0 725 510 A relates to a method for reducing signal peak values in a transmission device. Symbols which cause the peak value to be exceeded are reduced by adding an adjusting symbol to the symbol to be sent so that the magnitude of the symbol to be sent is reduced. The adjusting symbol is generated by using subchannels which are not used for the data transmission. For the subchannels vectors are generated which are added to the symbol in such a way that, when an inverse Fourier transform is performed, the contributions of the added vectors, peak values of those subchannels which are used for data transmission, are reduced.
It is the object of the present invention to provide a method for peak value reduction in single-carrier modulated or multi-carrier modulated digital transmitter signals which enables an effective and substantial reduction of the peak value in the transmitter signals to be made in a simple way.
The present invention is a method for peak value reduction in single-carrier modulated digital transmitter signals, in which (a) on the basis of a plurality of transmitter symbols contained in the digital signal, the peak value from the plurality of transmitter symbols is determined, and (b) if the peak value which is determined exceeds a predetermined value, at least one replacement symbol is generated which replaces the transmitter symbol assigned to this peak value, the replacement symbol reducing the peak value and being convertible into the original transmitter symbol without error using a coding which was used to create the transmitter symbols, where the generated replacement symbol has a small Hamming distance to the original transmitter symbol and is opposed antipodally or in magnitude and phase to the peak value assigned to the original transmitter symbol.
The present invention is a method for peak value reduction in multi-carrier modulated digital transmitter signals, in which (a) the peak value taking account of at least one transmitter symbol for each carrier frequency is determined, and (b) if the peak value exceeds a predetermined value, a replacement symbol is generated which replaces the transmitter symbol assigned to this peak value, the replacement symbol reducing the peak value and being convertible into the original transmitter symbol without error using a coding which was used to create the transmitter symbols, where the generated replacement symbol has a small Hamming distance to the original transmitter symbol and is opposed antipodally or in magnitude and phase to the peak value assigned to the original transmitter symbol.
According to a preferred embodiment of the present invention the replacement symbol is generated by inserting a bit error into the transmitter symbol which is to be replaced.
According to a further preferred embodiment of the present invention an arbitrary point in the complex plane is chosen as the replacement symbol such that the replacement symbol when it is received and converted into a transmitter symbol has a small Hamming distance to the original transmitter symbol.
According to a further preferred embodiment of the present invention the multi-carrier signal for modulating the transmitter signal comprises less than 50 carriers, preferably 10 to 20 carriers.
An advantage of the present invention is that the method cited above requires no additional outlay at the receiver of the digital signal since the replacement symbol can be converted without error into the original transmitter symbol using a coding which was used to create the transmitter symbol. A further advantage of the present invention is that the redundancy which is provided can be apportioned effectively between the correction of errors caused by channel noise and peak value reduction.
Yet a further advantage is that only one encoder/decoder set is required.
Furthermore, an advantage of the present invention is that implementation of the method according to the present invention and in particular the decoding of the transmitter signal which is sent does not involve any signalling of information between the transmitter and the receiver concerning the replacement symbol.
Yet a further advantage of the present invention is that, both for the case where one carrier is used and that where a plurality of carriers is used, there is no need for a special code for generating the replacement symbols since the channel decoding performs this task quite naturally.